The microwave energy moderator of the present invention and wraps, bags or vessels utilizing it have many uses. The material to be subjected to the moderated microwave energy does not constitute a limitation of the present invention and the moderator can be constructed so as to be used with electromagnetic energy of various wavelengths, as will be evident to one skilled in the art.
For purposes of an exemplary showing, the moderator and its various embodiments will be described in their application to vessels, bags or wraps for foods to be prepared in a microwave oven.
In recent years the use of microwave ovens to heat or cook foods has increased markedly both in the home and in commercial establishments. This is true for numerous reasons. For example, microwave ovens require no pre-warming, heating efficiently and result in energy savings. Many foods demonstrate a superior taste when prepared in a microwave oven and retain more of their nutritional components. Microwave ovens are perhaps best known for the speed with which they heat or cook and they offer both the homemaker and the commercial establishment rapid reheating of refrigerated pre-cooked foods.
Microwave ovens are not, however, without certain disadvantages. For example, heating or cooking by means of a microwave oven is so rapid that an error of several minutes (or sometimes less) can make the difference between a well done roast and a rare roast, or properly cooked foods and over-cooked foods. Each food product, itself, possesses characteristics having a marked influence on cooking or heating time. For example, such factors as the quantity of the food product to be heated or cooked, the size of the food product, the shape of the food product, its consistency and its dielectric properties all influence the rapidity and uniformity with which it will be heated or cooked in a microwave oven.
Furthermore, microwave ovens by different manufacturers differ in power outputs. Most domestic microwave ovens are produced with power outputs in the range of 600 watts to 1,000 watts at a nominal frequency of 2,450 MHz. The nominal wave length at this frequency is 12.2 cm. Another nominal frequency assigned to microwave cooking is 915 MHz with a nominal wave length of 33 cm.
Finally, the microwaves with the oven chamber tend in some places to reinforce each other and in other places tend to cancel each other. This phenomenon often results in hot and cold regions in food products being heated or cooked in a microwave oven. This effects non-uniform heating or cooking which is normally very undesirable.
All of the above noted factors result in the fact that heating or cooking with a microwave oven is generally more critical with respect to time and required attention than in conventional heating or cooking.
Prior art workers have attempted to solve some of the above noted problems by providing a variety of inventions directed towards improving the quality of microwave cooking. For instance, U.S. Pat. No. 3,219,460, issued Nov. 23, 1965, in the name of Eugene Brown, teaches a food container having several frozen foods therein, each requiring different cooking times. The container is made of microwave energy reflective material and is divided into compartments for each of the frozen foods. The container is provided with a microwave energy shielding cover. Those portions of the cover overlying each of the compartments are provided with microwave energy transmitting apertures, the number or configuration of which is selected to assure that the food product within each compartment is properly cooked or heated. A somewhat similar approach is taught in U.S. Pat. No. 3,547,661, issued Dec. 15, 1970, in the name of Peter N. Stevenson. This reference also teaches a container for selectively heating different items to different temperature levels simultaneously within a microwave oven. The vessel is generally microwave energy opaque, but opposite walls may be provided with areas of different microwave energy transmissivities. This is accomplished, for example, by making these opposite walls of microwave energy transparent material and coating them with aluminum. Various areas of these walls are provided with more or less aluminum coating (or no aluminum coating if maximum heating is to be achieved) corresponding to the various heating requirements of the food items these areas overlie.
U.S. Pat. No. 4,013,798, issued Mar. 22, 1977, in the name of Costas E. Goltsos, teaches another compartmented tray adapted to contain various food products. The tray is also provided with a specially formed shielding box, which box is provided with apertures so placed and sized as to control the quantity of microwave energy to which each of the food components is exposed within a given time period.
U.S. Pat. No. 4,027,132, issued May 31, 1977, to Melvin L. Levinson teaches a container for selectively defrosting and baking a frozen pie or pizza pie. The aparatus is constructed such that the crust of the pie is exposed to microwave radiation. The waste heat expended in the baking is used to heat the remainder of the pie which is shielded from the microwave radiation by a vapor-permeable, microwave-reflective member.
The above described prior art inventions are all static devices as opposed to the present invention which is a dynamic, thermally responsive microwave energy moderator which undergoes a dynamic change in its microwave energy transmissibility as its temperature is increased to a predetermined level or range.
Both static and dynamic microwave energy moderators are taught in the commonly owned copending application Ser. No. 854,941, filed Nov. 25, 1977, in the names of Thomas J. Flautt, Jr., Edward J. Maguire, Jr., and David L. Richardson and entitled MICROWAVE ENERGY MODERATOR. The static microwave energy moderators of this copending application are such that their relative degrees of transparency, transmissibility, attenuation and/or moderation of the microwave energy impinging thereon do not change substantially in use. Among other advantages, the use of the static moderators results in a substantially more uniform microwave energy field downstream thereof than the uniformity of the field would be absent such moderators. One of a number of examples of static moderators taught in this copending application comprises outer sheets of thermoplastic, microwave energy transmitting material and an intermediate sheet of microwave energy reflective material affixed or adhered to the outer thermoplastic sheets. The reflective sheet has a plurality of holes formed therein. The holes are so positioned and so sized as to render the moderator substantially transparent to microwave energy of a predetermined frequency range, yet sufficiently small and so spaced as to cause the microwave energy passing therethrough to be somewhat attenuated and/or moderated.
The dynamic embodiments of microwave energy moderators taught in this copending application are such that their relative degrees of transparency, transmissibility, attenuation and/or moderation of impinging microwave energy change substantially as the moderator is heated through a predetermined temperature range. Unlike the static embodiments and the dimensionally stable dynamic embodiments of the present invention, the dynamic embodiments of the copending application undergo substantial structural changes as they are heated to predetermined temperatures. One of a number of dynamic moderators taught in this copending application is similar to the static moderator just described with the exception that at least one of the thermoplastic sheets comprises a heat shrinkable thermoplastic film, which shrinks as it passes through a predetermined temperature range. When the film has shrunk, the effective sizes of the holes or apertures in the microwave energy reflective intermediate sheet are substantially reduced, since the shrinking action causes the microwave energy reflective sheet to crumple.
Both the static and dynamic microwave energy moderators of this copending application can be used as discrete microwave moderators, or they can be formed into wraps, bags, vessels, microwave oven liners, packages, containers or the like.
Another form of dynamic microwave energy moderator is taught in U.S. Pat. No. 4,144,435 issued Mar. 13, 1979 in the names of Clarence O. Clark, Robert L, DeAngelis, Kenneth F. Deffren, Thomas J. Flautt, Erwin A. Hofmann and Eugene Weinshenker. This copending application relates to various forms of vessels which are reflective to microwave energy. Each vessel has at least one aperture to permit the passage of microwave energy to its contents and a shielding device, responsive to a preselected internal temperature of the vessel contents, to close the at least one aperture to prevent further passage of microwave energy to the vessel contents.
The present invention is directed to a dynamic microwave energy moderator which may comprise a sheet-like laminate which is dimensionally stable and which transitions from a substantially transparent state with respect to microwave energy of predetermined frequency to a state wherein it is substantially less transparent to the microwave energy. This transition occurs in response to a predetermined temperature range through which the moderator passes. The sheet-like moderator of the present invention should be maintained generally planar for best operation and may be incorporated in microwave oven wraps or cooking vessels. For example, the moderator might constitute a cover for a disposable or reusable cooking vessel. It can also constitute a disposable element of a reusable vessel, being used as a disposable panel in the top, side walls, bottom or the like of the cooking vessel. Similarly, the moderator of the present invention can be used in the top, side walls, bottom or the like of a disposable cooking vessel. When used in conventional microwave oven cooking, the moderator of the present invention provides a number of benefits including more uniform doneness, reduced criticality of timing and reduced attention requirements.